The present invention relates to the deposition of chromium onto other metals. The use of chromium electrodeposited onto other metals is a well established industrial practice because of the wide need for the superior wear and corrosion resistance provided by chromium coated surfaces.
In the past chromium coating was achieved by the deposition of hexavalent chromium. This provided for hard, smooth coatings of chromium. However, undesirable waste products are created by the process of coating with hexavalent chromium and the hexavalent chromium coating process is energy inefficient.
There is a need for a coating system which provides the properties of hard chromium without the waste treatment problems or physical hazards associated with the deposition processes for hexavalent chromium.
Trivalent chromium deposits in the past have had a slightly darker appearance and have been softer than the hexavalent chromium coatings. Thick chromium coatings with hard wear resistant surfaces were not available from trivalent processes.
Trivalent chromium can not be plated directly in a simple electrolyte but requires highly complex systems with coordinating ligands including water and complexes. Chromium chloride has three kinds of complexes: [Cr(OH.sub.2).sub.4 Cl.sub.2 ]Cl.2H.sub.2 O in the green form; [Cr(OH.sub.2).sub.5 Cl]Cl.sub.2.H.sub.2 O in the blue-violet form; and [Cr(OH.sub.2).sub.6 ]Cl.sub.3 in the violet form. Dissolution of these forms in water gives an equilibrium mixture of the tetra, penta and hexa aqua complexes. The exact composition is dependent on pH, temperature and concentration. The equilibrium of these complexes is slow.